Since the mid 1930s, vehicles such as automobiles and light trucks have predominantly utilized hydraulic brake systems having a pedal operated master cylinder supplying pressurized hydraulic fluid to disk or drum braking devices at each wheel.
Early hydraulic brake systems utilized a single hydraulic fluid circuit supplying pressurized fluid from the master cylinder to all four corners of the vehicle. A break in the fluid circuit anywhere rendered the entire hydraulic brake system inoperative.
In order to prevent a total loss of hydraulic braking in the event of a failure of part of the system, failsafe hydraulic split brake systems were developed that provided two separate fluid circuits from the master cylinder, configured such that a failure of either of the two fluid circuits would still leave hydraulic brakes operative on at least two corners of the vehicle. In rear wheel drive automobiles and light trucks, one fluid circuit typically served the front wheels, and the other fluid circuit served the rear wheels, to provide a front/rear (F/R) failsafe hydraulic split system. Front wheel drive vehicles typically used a diagonal failsafe hydraulic split system, having one front corner and the diagonally opposite rear corner of the vehicle on one fluid circuit, and the other front corner and its diagonally opposite rear corner on the second fluid circuit. These failsafe provisions were incorporated into government regulations that required brake systems to be configured such that a single failure of the braking system would still leave the brakes on at least two corners of the vehicle operational.
In the years since hydraulic brake systems became the norm, many additional features have been added to further enhance safe operation and optimize vehicle performance. Modem brake systems often include a booster that amplifies force exerted on the brake pedal, to provide power brakes that allow a person operating the vehicle to control the brakes with significantly less force on the brake pedal than is required in a non-boosted brake system. Anti-lock brake systems (ABS) were developed in which valves controlling fluid flow to each corner of the vehicle were pulsed, in response to signals received from rotation sensors monitoring each wheel, to preclude locking the brakes on slippery road surfaces. Traction control systems (TCS) were added that controlled both the brakes and the engine throttle setting to improve traction and handling of the vehicle during maneuvers, such as acceleration or turning, when the brakes are not being applied by the operator. Vehicle dynamics control (VDC) further advanced the level of sophistication of brake systems to utilize a number of sensors throughout the vehicle, and a more advanced onboard computer with higher throughput, to monitor forces acting on the vehicle, together with inputs indicating operational commands from the operator applied to the steering, braking, and drive systems. VDC analyzes the data received from the sensors and coordinates operation of the various elements of the vehicle brake system, power-train, and suspension to provide enhanced vehicle safety or performance of the vehicle.
The addition of all of these enhancements has made hydraulic brake systems more complex. Numerous valves, sensors, and electronic control components are required.
Recent advances in technology have made it feasible to develop a brake system that utilizes electrically actuated brakes, rather than hydraulic brakes, on at least the rear corners of a vehicle. Utilizing electrically actuated brakes allows a number of the components currently required in fully hydraulic brake systems to be eliminated, particularly the interconnecting brake pipes, thereby resulting in ease of installation for the vehicle assembly plants. Hydraulic brake systems must be carefully filled with fluid, to preclude trapping air in the hydraulic lines that would interfere with operation, thereby increasing manufacturing complexity, time and cost. Electrically actuated brakes eliminate the manufacturing complexity, time and cost associated with filling the fluid lines in hydraulic brake systems. Electrically actuated brakes also provide opportunities for additional functionality, such as electrical park brake capability, and improved operational performance in modern brake systems.
Modern brake systems rely heavily on electronic controls to coordinate and control functions such as ABS, TCS, and VDC, making it logical to move toward an all-electric brake system. Most modern automobiles and light trucks utilize a 12 volt electrical system. While 12 volt electrically actuated brakes are currently feasible for rear brakes, it is generally accepted by those having skill in the art, that front electrically actuated brakes would need to be operated at a higher voltage, such as the proposed 42 volt systems now in development, to be able to cost-effectively manage the higher power levels required. The higher voltage is required because the front brakes handle a significantly higher percentage of the braking load than the rear brakes. As a generally accepted rule of thumb, the front brakes provide ⅔ of the stopping power, and the rear brakes provide the remaining ⅓ when both the front and rear brakes are operating for a moderate vehicle deceleration.
Providing a hybrid brake apparatus having electrically actuated brakes on the rear of a vehicle and hydraulic front brakes reduces the work load on the hydraulic brake system by approximately ⅓, thereby allowing components in the hydraulic portion of the hybrid system to be downsized, thereby saving cost, space, and weight. Reducing the required work on the hydraulic portion of the system may also make it possible to more readily achieve a “car-like” feel to the brake pedal on light duty trucks.
A hybrid brake apparatus having front hydraulically actuated brakes and rear electrically actuated brakes presents previously unforeseen challenges to failsafe operation, which were not presented by all-hydraulic brake systems. These new challenges necessitate new approaches to providing failsafe construction and operation of a hybrid brake apparatus.
What is needed, therefore, is a hybrid brake apparatus having front hydraulically actuated brakes and rear electrically actuated brakes, that meets or exceeds the safety requirements previously applied to all-hydraulic brake systems, and providing performance capabilities that are as good as or better than prior all-hydraulic brake systems. Such a hybrid brake apparatus should also be applicable to a brake system including functions such as ABS, TCS, and VDC. It is also highly desirable that the hybrid brake apparatus have the same or better operational feel to a driver applying force to the brake pedal, as a conventional all-hydraulic brake system.